Before splicing two optical fibers to each other, a proper mutual alignment of the fibers is essential, since this will minimize the optical attenuation for light propagating in the fibers and through the splice. In the particular case of aligning two PM fibers to each other, special consideration of the geometry of the fibers has to be made. Like conventional fibers, commercially available PM fibers have a core region and a surrounding cladding, the cladding having generally a circular-cylindrical outer surface. However, the distribution of the refractive index over a cross-section perpendicular to the longitudinal axis of PM fibers is not circular-symmetric with respect to the fiber axis unlike the conventional case.
For splicing PM fibers to each other an important issue is therefore to achieve a good angular alignment or azimuthal alignment, so that, for two PM fibers, regions of equivalent refractive indices are as close as possible to each other at the two opposite fiber end faces, located closely at each other, at which the fibers are to be spliced to each other. Two basic methods are frequently used for the angular alignment, the so-called active and passive alignment methods. For the active alignment method, a highly polarized light source, a polarization extinction ratio (PER) meter and an apparatus provided with optical fiber rotators are needed. The PER is defined as the optical power ratio in dB form measured along two main optical axes. The angular alignment can be achieved by maximizing the value of PER while rotating one fiber end with respect to the other at the splicing point. A typical apparatus using the active method for angular alignment of PM fibers was disclosed in 1992, see U.S. Pat. No. 5,156,663, Oct. 20, 1992, for Keinichiro Itoh et al.
The passive alignment method is performed locally at the splice point with the assistance of digital imaging techniques in an automated fusion splicer. Several different techniques have been developed for passively aligning PM fibers. A method using an interference pattern to determine the polarization axes of PM fibers was disclosed in 1994, see U.S. Pat. No. 5,323,225, Jun. 21, 1994, for Richard B. Dyott et al. A method using the photoelastic effect to determine the polarization axes of PM fibers was disclosed in 1995, see U.S. Pat. No. 5,417,733, May 23, 1995, for Laurence N. Wesson. Methods of intensity profile analysis have also been proposed, e.g. the fiber side-view method, see H. Taya, K. Ito, T. Yamada and M. Yoshinuma, “New splicing method for polarization maintaining single mode fibers,” Conf. on Optical Fiber Communication (OFC'89), THJ2, 1989, and H. Taya, K. Ito, T. Yamada and M. Yoshinuma, “Fusion splicer for polarization maintaining single mode fiber”, Fujikura Technical Review, pp. 31–36, 1990, and the fiber end-view method, see U.S. Pat. No. 5,147,434, Sep. 15, 1992, for K. Itoh, T. Yamada, T. Onodera, M. Yoshinuma and Y. Kato, “Apparatus for fusion splicing a pair of polarization maintaining optical fibers”, and U.S. Pat. No. 5,156,663, Oct. 20, 1992, for K. Itoh, T. Yamada, T. Onodera, M. Yoshinuma and Y. Kato, “Apparatus for fusion splicing a pair of polarization maintaining optical fibers”. More advanced techniques, see Wenxin Zheng, “Automated Fusion-Splicing of Polarization Maintaining Fibers”, IEEE J. Lightwave Tech., Vol. 15, No. 1, 1997, e.g. the combination of the polarization observation by lens effect tracing (POL)-profile with the method of POL-correlation for directly and indirectly determining the angular offset of PM fibers, have also been disclosed, see Swedish Patent No. 9300522-1, March, 1993, inventors Wenxin Zheng et al., U.S. Pat. No. 5,572,313, Nov. 5, 1996, for Wenxin Zheng et al., U.S. Pat. No. 5,758,000, May 26, 1998, for Wenxin Zheng et al., and the published International Patent application No. WO 01/8633 for Wei-Ping Huang et al. These techniques were very successfully employed in automated arc fusion splicers for the most common PM fibers then available in the market, e.g. the Panda and the Bowtie fibers.
Recently, elliptical-core fibers have attracted great interest in construction of communication systems, e.g. in constructing erbium-doped PM fiber amplifiers and optical fiber sensors. Unfortunately, the existing alignment techniques, see the above-cited patents on POL-profile methods, can hardly generate stable and consistent results of angular alignment for the elliptical-core type due to primary technical limitations. For example, the methods are not sensitive enough to accurately measure the small variations in the intensity profiles when rotating the fibers. Thus, there is a need in the art to improve the existing alignment techniques, in particular those based on the POL-profile, in order to be capable of handling all types of PM fibers.
In particular, these problems appear in illuminating each fiber from a side thereof and regarding the fiber as a cylindrical lens, observing the light intensity variations in the focal plane along a line perpendicular both to the longitudinal axis of the fiber and to the propagation direction of the illuminating light source. Typically, the intensity has a central peak that varies in height when the fiber is rotated about its longitudinal axis, see the Swedish Patent No. 9300522-1 and the published International Patent application No. WO 01/8633 cited above. In this context it is interesting to calculate the light contrast, h, which is the difference in intensity between the central peak and the surrounding region. The profile of the light contrast is obtained as the variation of the light contrast as a function of the angle of rotation, i.e. the azimuthal angle.
A highest possible contrast of h-values, i.e. of the difference between the maximum and the minimum h values in the profile of the light contrast, is essential to ensure a high quality of the contrast profile. It turns out that, for PM fibers of elliptical core type the contrast of the h-values is usually less than 10 grey scale levels as measured in a typical automated fusion splicer. Thus in this case, the light contrast profile becomes extremely sensitive to the adjustment of the optical image system of the splicer used.